An internal gear pump constituted by accommodating a pump rotor, which is formed by combining an inner rotor having n teeth and an outer rotor having (n+1) teeth and eccentrically disposing the rotors relative to each other, within a rotor chamber of a housing is used as, for example, an oil pump for lubricating a vehicle engine or for an automatic transmission (AT).
One example of such an internal gear pump is disclosed in Patent Literature 1 described below.
In the internal gear pump disclosed in Patent Literature 1, a trochoidal curve is first drawn along the locus of a fixation point located distant from the center of a rolling circle, which rolls along a base circle without sliding, by e. Then, an envelope of a group of locus circles each having its center on the trochoidal curve serves as a tooth profile of the inner rotor.
A tooth profile of the outer rotor is formed by using the locus of a group of tooth-profile curves of the inner rotor. Specifically, the center of the inner rotor revolves by one lap along a circle having a diameter of (2e+t) and centered on the center of the outer rotor (e being an amount of eccentricity between the inner rotor and the outer rotor, and t being a tip clearance between the inner rotor and the outer rotor at a theoretical eccentric position) while the inner rotor rotates (1/n) times. An envelope of a group of tooth-profile curves of the inner rotor at that time serves as the tooth profile of the outer rotor.